Illinois State Climatologist Office

First Half of November Warmer than October

The state-wide average temperature for November 1-15 was 50.1 degrees, 5.7 degrees above normal, and 0.3 degrees warmer than October. Precipitation across the state was extremely light, only 0.09 inches, until rains arrived during the day on November 15.

Second Wettest October on Record

The state-wide precipitation for October was 8.90 inches. This is 6.0 inches above normal. Based on preliminary data, this is the second wettest October on record. The wettest October was 1941 with 9.23 inches. The heaviest rains fell in the southern half of the state with totals ranging from 6 to 12 inches. Here are more details on October.

Statewide temperatures for October have been 4.8 degrees below normal, making this the seventh coldest October on record. Statewide records of temperature and precipitation go back to 1895.

Check out the map of Midwestern stations that set or almost set their October rainfall records, courtesy of the Midwestern Regional Climate Center.

Cool, Wet Growing Season Delays Harvest

According to statistics from the Illinois office of USDA's National Agricultural Statistics Service, the percent of corn harvest as of October 25 was only 14 percent. This is by far the slowest harvest for corn by this date in Illinois when compared to records dating back to 1980 (click to enlarge graph).

The first weather challenge for the 2009 growing season was the much above normal rainfall in March-June, which slowed planting and related field work. Statewide rainfall during this period was 21.0 inches, 5.5 inches above normal and the 7th wettest March-June on record.

This was followed by the coldest July on record when the statewide average temperature was only 70.3 degrees, 5.1 degrees below normal. August was cool as well, 2.1 degrees below normal. Rainfall was closer to normal in July-September with the statewide average at 10.4 inches, only 0.5 inches below normal. However, this was followed by the second wettest October's on record (see below).

September Brings Uneven Rainfall across State

The state-wide precipitation for September was 2.4 inches, 0.8 inches below normal. This breaks the string of months with above-normal precipitation that started in February 2009 and ended in August 2009. While much of southern and western Illinois received above normal precipitation, northern and eastern Illinois were drier than normal.

Rainfall was heaviest in southeast Illinois with Lawrenceville reporting 10.53 inches for the month. Many sites in northern and eastern Illinois reported totals of less than an inch of rain.

Temperatures were slightly below normal with a state-wide temperature average of 65.8 degrees, compared to a normal of 66.2 degrees.  As of September 30, no station had reported a freezing temperature (32 degrees or less). See the press release.

Tracking the First Fall Freeze Across the Midwest

The Midwestern Regional Climate Center has an interesting map showing the progression of frost across the Midwest, in other words, places that have already hit 32 and 28 degrees this fall. The map is updated daily by 11:30 am. Speaking of frost, here are the average frost dates in Illinois. As of September 28, the USDA report for Illinois stated that only about 2 percent of the corn and 1 percent of the soybeans were harvested - well behind the 5-year average.

Impacts of El Niño on Temperature and Precipitation

The NOAA Climate Prediction Center has identified a weak El Niño event in the Pacific Ocean. This event is expected to strengthen and last through this winter (2009-2010). Here is a series of maps on the historical impacts of El Niño on monthly temperature and precipitation (pdf).

In general, El Niño produced warmer-than-normal temperatures in September and during December-March. In contrast, cooler-than-normal temperatures prevailed in August and April-May. The impact on monthly precipitation was both weaker and less consistent. Somewhat wetter conditions prevailed in August, October, and December while drier conditions were found in September. 

Illinois Agricultural Disaster Declaration

On September 13, 2009, 88 counties in Illinois were declared as natural disaster areas because of too much rainfall during the first half of the growing season. See press release. Rainfall during the April-July period totaled 20.94 inches, 4.74 inches above normal. Here are the preliminary rainfall totals for individual sites in Illinois during this period.

Cool, Wet Summer in Illinois

Based on preliminary statewide numbers, this is the 11th coolest and 8th wettest summer since statewide record began in 1895. The average temperature for June-August was 71.4 degrees, 2.4 degrees below normal. The total precipitation for June-August was 15.2 inches, 3.6 inches above normal.

The coolest summer on record was 1915 with 69.3 degrees. In recent years, 1992 was ranked third with 70.2 degrees and 2004 was ranked fourth with 70.4 degrees. The wettest summer on record was 1993 with 18.3 inches of rain.

The statewide August average temperature was 71.0 degrees, 2.6 degrees below normal and the 15th coolest August on record. The statewide August total precipitation was 5.0 inches, 1.4 inches above normal and the 17th wettest August on record.

Departure map links for summer temperature and precipitation and for August temperature and precipitation

This may be the cloudiest summer on record. According to long-term records of sunshine kept by Frank Wachowski, weather observer for Chicago/Midway, the percent sunshine this summer was 53 percent, a new low for Chicago. Normal summer sunshine is 67 percent.

Average Wind Speed Maps

Link: New web page on average wind speed in Illinois

El Niño Not Likely to Affect First Fall Freeze

An Illinois State Water Survey press release from last week indicates that the developing El Niño in the Pacific Ocean will likely not affect the first freeze this fall in Midwest. Even cool summer weather does not appear to change the odds of an early freeze.

The average first fall freeze (32F) can be found here. Generally, it is early October in northern Illinois, mid-October in central Illinois, and late October in southern Illinois. However, the actual freeze date from year to year can vary by a week or two from those dates.

Coldest July on Record in Illinois

Statewide temperature: Based on preliminary data for Illinois, the statewide average temperature for July was 70.4 degrees, 5.3 degrees below normal. This ranked as the coldest July statewide by more than a degree (see list below for five coldest Julys). Statewide records go back to 1895. While cooler than normal temperatures prevailed throughout the Midwest and East Coast in July (click on map to the right), the Southwest and West were warmer than normal.

Coldest Julys in Illinois

1. 2009 - 70.4°F
2. 1924 - 71.5°F
3. 1967 - 72.0°F
4. 1950 - 72.4°F
5. 1915, 1947 (tie) - 72.5°F

Lack of hot weather: While southern Illinois reported a few days with temperatures in the 90s in July, much of the rest of the state saw none. The normal range of days in the 90s is typically between 5 in northern Illinois to 15 in southern Illinois. Click on map to the left. In Champaign-Urbana this was only the fourth time in its 121 year history that the thermometer failed to reach 90°F in July. The other three times were 2000, 2007, and 2008.

Consistently cold: Another remarkable feature of July was the consistency of the colder than normal temperatures. On a statewide basis, every day was below normal except for July 11. Click on map at right. In addition, 33 record daily low temperatures as well as 218 record low high temperatures were set for July. New records from around the US can be found on the National Climatic Data Center web site here.

Fewer degree days: One clear impact was lowered energy demand for cooling homes and businesses during July; cooling degree days (CDD) were down 48 percent from normal. Meanwhile, modified growing degree days (a measure of corn development) were down 17 percent from normal, suggesting less favorable growing conditions. Degree day definitions can be found here.

Outlook: Historically, a cool July is most often followed by a cool August. The specific odds on August temperatures after a cool July are 45 percent cooler-than-normal, 34 percent near-normal, and 21 percent above-normal. The NOAA Climate Prediction Center indicates that cooler than normal temperatures are likely to prevail for August across Illinois, and for the northern two-thirds of Illinois for the August-October period.

Statewide precipitation: The statewide average precipitation for July was 4.9 inches, 1.1 inches above normal. Precipitation since January 1 stands at 29.0 inches, 5.6 inches above normal. See map for more details.

Climate Events of 2009 in Illinois

Find the significant climate-related events of 2009 in Illinois here.

• June - another wet month
• Hot, humid weather arrives
• Fifth Wettest Sprint on Record
• Sixth Wettest April on Record
• March - Wet in the North, Dry in the South
• Where is Spring
• Average Last Spring Frost
• February - More Rain, Less Snow
• Illinois Temperature Roller Coaster
• Former Champaign-Urbana weather observer passes away
• Cold January
• Arctic Air Grips Illinois

Climate Events in Illinois Archive

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